The present invention relates to a method for preservation of wood using a wood preservative based on one or more compounds of at least one metal of the 1st and/or 2nd subgroup of the periodic system, where the preservative is at least partially fixed in the wood for purposes of reducing the danger of leaching. In another aspect, the invention relates to the preserved wood with reduced danger of leaching that can be obtained in accordance with the method of the invention.
The life span of wood used for construction purposes, telegraph poles, railroad ties and the like, can, as is well known, be increased by treating it with a preservative. Such preservative agents have a toxic effect on the fungi and insects that damage wood. The preservatives are in many cases inorganic metal compounds such as compounds of chromium, copper, and arsenic. More recently, attempts are being made to avoid the use of Cr- and As-containing preservatives, because of their toxicity. Water-soluble compounds of these metals are introduced at least into the surface regions of the wood by means of substantially known impregnation processes. The initially water-soluble metal compounds are converted at least partially into sparingly soluble compounds through oxidative processes and by interaction between the metal compounds and components of the wood. This process progresses slowly and incompletely. Through intensive contact with water, metals are partly leached out, and the preservative effect diminishes at the same time.
Impregnating wood exclusively with a copper salt solution and undertaking an at least partial fixation of the copper by subsequent heat treatment using steam or hot air, for example, is also known. This process has high energy requirements and reduces the capacity of the treatment plant for wood preservation to a considerable extent.
Accordingly, an object of this invention is to improve methods for preserving wood, and particularly to reduce the danger of leaching.
The above and other objects of the invention can be achieved by impregnating wood with a wood preservative based on metal compounds of metal selected from the 1st and/or 2nd subgroups of the Periodic Table of Elements (Groups IB and IIB) and then subsequently fixing. In accordance with a feature of the invention, it should be ensured that the fungicidal effect of the metal compounds is not adversely affected by the fixing step. A feature of the invention is aimed at not only effectively reducing leaching of metal from wood treated with a copper-containing preservative by means of a post-treatment, but also of increasing the fungicidal effect.
It is known that many metals, including those selected from the group consisting of the 1st and 2nd subgroups of the periodic system, form sparingly soluble salts with trimercapto-s-triazine, hereinafter called TMT. Accordingly, in wastewater treatment technology heavy metal compounds are precipitated by a treatment with a water-soluble salt of TMT. The principle of formation of sparingly soluble salts of heavy metals of the 1st and 2nd subgroups with TMT can also be applied to the fixing of heavy metals introduced into wood by means of an impregnation through treatment with an aqueous solution of a TMT salt. Salt formation of a corresponding kind is also possible with a dithiocarbamate or trithiocarbonate.
It was also found that fixing Cu with TMT is possible, but the fungicidal effect toward certain fungi is largely lost, and thus the wood is left open to attack by microorganisms. However, it was surprisingly found that through the presence of zinc in a preservative, where the zinc can even have been introduced through a separate impregnation, the fungicidal effect of this combination (Cu+Zn) and of other zinc-containing combinations with another metal of the 1st or 2nd subgroup of the periodic system is not reduced, but rather is increased.
A method was found for preserving wood that includes impregnating the wood with a wood preservative agent based on one or more compounds of at least one metal selected from the group consisting of the 1st subgroup (Group IB) of the Periodic Table of Elements, the 2nd subgroup (Group IIB) of the Periodic Table of Elements and mixtures thereof, and then fixing the metal for purposes of reducing the danger of leaching. The method is carried out by (i) impregnating the wood with a preservative containing a water-soluble zinc compound, or (ii) impregnating wood that has been previously impregnated with a zinc-free preservative that contains at least one metal compound of the 1st and/or 2nd subgroup with a second preservative that contains at least one water-soluble zinc compound. The wood impregnated in accordance with either (i) or (ii) is then impregnated with a water-soluble solution of a salt of trimercapto-s-triazine (TMT), a water-soluble dithiocarbamate or trithiocarbonate. In accordance with preferred embodiments, the use of a copper- and zinc-containing preservative in the impregnation step and a post-treatment with a solution of a TMT salt is particularly preferred. Besides Cu and Zn salts, Ag and Hg salts in particular are suitable as preservative agents. The further embodiments concern TMT-containing systems, but they can also be extended to other organosulfur compounds.
According to a first embodiment the preservative to be used contains a water-soluble zinc compound. In the second impregnation with a water-soluble TMT salt that follows the first impregnation a sparingly soluble Zn-TMT salt forms. Due to this, the danger of leaching is reduced.
Preferably, the preservative contains, in addition to a zinc compound, one or more compounds of one or more other metals of the 1st and/or 2nd subgroup of the periodic system. It is especially preferred that the preservative contain a copper and a zinc compound, such as a water soluble salt. Expediently, these are the sulfates, chlorides, nitrates and acetates of the said metals.
According to another embodiment of the invention, wood that has already been preserved with a compound of a metal of the 1st and/or 2nd subgroup of the periodic system, except for zinc, can be impregnated with a solution of a zinc salt for purposes of introducing zinc. This additional impregnation step is conducted so that zinc is introduced at least into the surface regions of the wood.
The one- or two-step impregnation for purposes of introducing the metal compounds into the wood is carried out in a substantially known way. The term xe2x80x9cimpregnationxe2x80x9d is understood to mean treatments under normal pressure, excess pressure and preferably reduced pressure. The term includes immersion methods, where the wood is put into a solution containing the preservative as well as methods involving spraying the preservative onto the wood and letting the preservative work itself into the wood. With the latter embodiment only preservation in the surface regions is usually achieved.
The metal compounds introduced into the wood by one- or two-step impregnation are fixed by impregnation with a solution of a TMT salt. Here, too, the term xe2x80x9cimpregnationxe2x80x9d as used herein includes the embodiments mentioned above.
Suitable water-soluble salts of TMT are lithium, sodium, potassium and ammonium salts. As trivalent mercapto compounds the TMT salts can have one, two or three cations. The sodium salts of TMT are particularly preferred; in view of solubility, the di- and especially the trisodium TMT salts are used. These water solvable salts are well known and any suitable one may be used in accordance with the present invention.
The concentration of the TMT salt in the solution used for impregnation can lie in a wide range, in particular in the range from 0.01 wt % up to the saturation concentration of the salt. A usage concentration in the range of 0.05 to 5 wt % is preferred and one in the range of 0.1 to 1 wt % is especially preferred.
The usage concentration of the solution used in the preservation step (the 1st impregnation step), which solution contains at least one metal compound, preferably a zinc compound, and a compound of another metal of the 1st or 2nd subgroup of the periodic system, especially a copper compound, lies in a substantially known range. Expediently this means a metal concentration each in the range from 0.01 to 10 wt %, preferably 0.1 to 5 wt %.
It was found, as shown in the examples, that in the case of wood impregnated beforehand with one or more generic metal salts and treated by an impregnation with the TMT salt does undergo a certain change of color caused by a leaching treatment, but the fungicidal effect toward the test fungus Tyromyces placenta is completely retained.
As follows from the comparison examples (experiments without TMT treatment) the preservative effect toward the test fungus Tyromyces placenta is lost in a corresponding leaching of preserved wood treated with Cu, Zn or Cu+Zn but not with a TMT salt. After leaching, wood treated with Cu shows a slight reduction of the fungicidal effect toward the test fungus Coniophora puteana, while wood preserved with Zn shows a clear reduction, and wood preserved with Cu+Zn shows a moderate reduction.
Wood treated with a Cu compound and a TMT salt (Cu/TMT) has little resistance toward the test fungus Coniophora puteana, and a clear loss of weight occurs. In contrast, Zn/TMT-treated wood shows clearly higher resistance. Surprisingly, the combination Cu+Zn/TMT shows a particularly high resistance toward this test fungus. Treatment of wood preserved with Cu and Zn using a TMT salt thus leads to a surprisingly increase of the resistance of the wood to fungal attack.